An open center hydraulic circuit (for example, see Patent Document 1) and a load sensing hydraulic circuit (for example, see Patent Document 2) have been used to drive a boom cylinder, an arm cylinder, a bucket cylinder, or the like of a small hydraulic shovel.
The open center hydraulic circuit however always requires a maximum flow rate during an operation and thus has a large power loss especially during traveling at a very low speed. Meanwhile, the load sensing hydraulic circuit causes pressure interference during a combined operation and thus has poor operability and a large power loss. Moreover, both the hydraulic circuits are not able to collect energy by the cylinder operated by a gravitational force.